The visual field of the human eye is not limited to external objects, but instead includes visible effects which arise from within the optical system itself, otherwise known as entoptics. One may be familiar with entoptics in the form of floaters or migraine auras, as well as the field of changing colors and shapes borne of gentle pressure applied to the back of shut eyelids. Through research on the human optical system, the basic morphological components of these visual phenomena have been classified into a standardized taxonomy. Because the naturalistic paintings of Dutch artist Piet Mondrian's early career carry a strong resemblance to the forms within this taxonomy, his works are suited to an entoptic-based analysis. In the past, such an approach has been used in the examination of prehistoric cave painting, but has only found limited application with modern artworks. By investigating the entoptic presence in the paintings of Mondrian, I therefore hope to demonstrate the wider applicability of entoptics in art historical analysis, as well as provide greater support for neurological models of image making.